NdFeB system sintered magnets were discovered in 1982 by Sagawa (one of the present inventors) and other researchers. The magnets exhibit characteristics far better than those of conventional permanent magnets and can be advantageously manufactured from Nd (a kind of rare-earth element), iron and boron, which are relatively abundant and inexpensive materials. Hence, NdFeB system sintered magnets are used in a variety of products, such as driving motors for hybrid or electric cars, battery-assisted bicycle motors, industrial motors, voice coil motors used in hard disk drives and other apparatuses, high-grade speakers, headphones, and permanent magnetic resonance imaging systems. NdFeB system sintered magnets used for those purposes must have a high coercive force HcJ and a high maximum energy product (BH)max.
It has been known that the coercive force of NdFeB system sintered magnets can be improved by making Dy, Tb or other heavy rare-earth elements RH present inside the magnet, since those elements make reverse magnetic domains less likely to develop when a magnetic field opposite to the direction of magnetization is applied. The reverse magnetic domain has the characteristic that it initially develops in a surface region of a main phase grain and then spreads into inner regions as well as over the neighboring main phase grains. Therefore, to prevent the initial development of the reverse magnetic domain, RH only needs to be present in the surface region of the main phase grain, whereby the development of the reverse magnetic domain on the surface of the main phase grain can be prevented. However, an increase in the RH content lowers the residual magnetic flux density Br, which leads to a decrease in the maximum energy product (BH)max. Accordingly, to increase the coercive force (i.e. to make the reverse magnetic domain less likely to develop) while minimizing the decrease in the maximum energy product (BH)max, it is desirable to make RH present at higher concentrations in the surface region of the main phase grain than in the inner regions.
One method for making RH present in a NdFeB system sintered magnet is a “single alloy method”, in which RH is added to a starting alloy in the step of preparing the alloy. Another method is a “binary alloy blending technique”, in which a main phase alloy which does not contain RH and a grain boundary phase alloy to which RH is added are prepared as two kinds of starting alloy powder, which are subsequently mixed together and sintered. Still another method is a “grain boundary diffusion method”, which includes the steps of preparing a NdFeB system sintered magnet as a base material, putting RH to the surface of the base material by application, deposition or another process, and heating the base material to diffuse RH from the surface of the base material into inner regions through the grain boundaries inside the base material (Patent Literature 1).
Among those methods, when the single alloy method is chosen, the starting alloy powder already contains RH uniformly distributed in its main phase grains, so that a sintered magnet created from this powder inevitably contains RH in the main phase grains. Therefore, the sintered magnet created by the single alloy method has a relatively low maximum energy product while it has a high coercive force. In the case of the binary alloy blending technique, the largest portion of RH will be held in the surface regions of the main phase grains. Therefore, as compared to the single alloy method, this technique can reduce the amount of decrease in the maximum energy product. Another advantage over the single alloy method is that the used amount of the rare metal RH is reduced.
In the case of the grain boundary diffusion method, RH attached to the surface of the base material is diffused into inner regions through the grain boundaries liquefied by heat in the base material. Since the diffusion rate of RH in the grain boundaries is much higher than the rate at which RH is diffused from the grain boundaries into the main phase grains, RH is promptly supplied into deeper regions of the base material. By contrast, the diffusion rate from the grain boundaries into the main phase grains is low, since the main phase grains remain in the solid state. Using this difference in the diffusion rate, the temperature and time of the heating process can be regulated so as to realize the ideal state in which the Dy or Tb concentration is high only in the vicinity of the surface of the main phase grains (grain boundaries) in the base material while the same concentration is low inside the main phase grains. Furthermore, since the heating temperature in the grain boundary diffusion process is lower than the sintering temperature, the melting of the main phase grains is less likely to occur than in the case of the binary alloy blending technique, so that the penetration of RH into the main phase grains is more effectively prevented than in the binary alloy blending technique. Therefore, the amount of decrease in the maximum energy product (BH)max can be made smaller than in the case of the binary alloy blending technique. Another advantage over the binary alloy blending technique is that the used amount of the rare metal RH is reduced.
There are two different methods for producing NdFeB system sintered magnets: a “press-applied magnet-production method” and a “press-less magnet-production method.” In the press-applied magnet-production method, which is a conventionally and commonly used method, fine powder of a starting alloy (which is hereinafter called the “alloy powder”) is placed in a mold, and a magnetic field is applied to the alloy powder while pressure is applied to the alloy powder with a pressing machine, whereby the creation of a compression-molded body and the orientation of the same body are simultaneously achieved. Then, the compression-molded body is removed from the mold and heated to be sintered. In the press-less magnet-production method, which has been discovered in recent years, alloy powder which has been put in a predetermined filling container is oriented and sintered as it is held in the filling container, without undergoing the compression molding (Patent Literature 2).
The press-applied magnet-production method requires a large-size pressing machine to create a green compact. Therefore, it is difficult to perform the processes from the filling through the sintering in a closed space. By contrast, the press-less magnet-production method has the advantage that it does not use a pressing machine and therefore allows the aforementioned processes to be performed in a closed space.